Obtaining a smooth result after modelling
You might have flipped normals on some of the faces.
Select all faces and hit CTRL+N to make the normals consistent. 

Also, make sure to check the mesh for disconnected geometry; select the object in Edit mode and click Mesh > Clean up > Merge by Distance. Find a value where the geometry of your model isn't affected negatively. If any vertices are removed you'll see this in the viewport toolbar.